Operators of wireless networks face a number of challenges in cost-effectively deploying networks resources to meet recent dramatic increases in the demand for total data capacity. This demand is being driven by the introduction of data intensive applications for smart phones, and new mobile devices with video capabilities, which in turn drive the introduction of additional data intensive applications. For example, in 2009, introduction of the iPhone® by one operator in the United States resulted in a sudden massive increase in the total traffic volume, with resultant stress on their network resources to provide the required cell site capacity to satisfy increased user demand. Other operators are seeing similar trends as they follow suit. Although cell splitting, with deployment of small cells, is an attractive option to increasing capacity, existing high capacity backhaul solutions depend on fibre and microwave and are costly to implement.
Operators have limited options to meet the increasing capacity demand with existing network technologies. If they have unused spectrum, the easiest method is to add carriers to increase the total RF bandwidth and hence the aggregate capacity of their cell sites. In many cases this can be cost effective. If they have deployed multi-carrier radios then increasing the carrier count does not require additional radios or antennas to be deployed. The disadvantage is that the additional carriers do not increase the Uplink speed since this is effectively limited by the path loss of the large cell and the limited energy per bit which a user terminal can generate.
Another option is to migrate to more spectrally efficient technologies, i.e. migration from current 3G technologies (e.g. based on CDMA and UMTS) towards next generation 4G technologies). Currently, the 3GPP LTE (Long Term Evolution) standard, which is based on MIMO/OFDMA (Multiple Input Multiple Output/Orthogonal Frequency Division Multiple Access), has emerged as the technology of choice and many operators are planning their migration from CDMA or UMTS to LTE over the next few years. Although the LTE technology is based on OFDM/MIMO, the uplink performance at the cell edge is not greatly increased, since this is still limited by the energy/bit that is required to compensate for the large path loss and the limited power which a UE (User Equipment?) transmitter is able to generate.
Moreover, as operators roll out 4G networks, they are faced with a delicate balancing act. They must invest heavily in infrastructure for a new air interface knowing that the initial subscriber density will be very low and their investment will not create significant amounts of revenue for several years. Most operators would expect their 4G investments to generate a net loss until a minimum subscriber density is achieved. To minimize the impact, an operator would likely choose to implement 4G in dense urban centers initially knowing that they will achieve a critical subscriber density relatively fast, and as these sites become profitable they would extend the coverage to increasingly less populated, less profitable areas. Although such a cautious deployment method makes sense, inter-operator competition for footprint may force operators to be more aggressive, take more risk, and deploy 4G aggressively in an effort to gain market share.
Cell splitting to increase the frequency reuse is a more powerful method and is an option even if an operator has used it entire available spectrum. The total aggregate capacity of the network increases in proportion to the number of cells. Furthermore, the user experience improves greatly since the smaller cell radius and lower propagation loss between the UE and BTS (Base Transceiver Station) means that the terminal needs to send less energy/bit and as a result can transmit over a larger bandwidth. Also, higher order modulations can be used given that a stronger signal results in a better Signal to Noise Ratio (SNR), which results in a more spectrally efficient communication link. For a fairly dense sub-urban neighbourhood the path loss exponents can be in the range of 3.5 to 4, which is to say that the path loss increases to the 4th power of the distance. So, to maintain a certain SNR at the receiver, if the distance reduces by half, the transmitter would only need to transmit (½)4= 1/16. Alternatively, for a given UE transmit power, the UE would now be able to transmit 16 times more bandwidth for a given desired SNR at the opposing receiver, which is a tremendous improvement in uplink performance. Given that Cell splitting increases both the aggregate network capacity, and the achievable uplink and downlink data rates, this option offers a very attractive deployment scenario for both existing 3G networks and the emerging 4G, LTE and WiMax networks and is expected to be a primary focus of wireless operators over the coming decade. This trend is also giving rise to a large demand for smaller lower power cell sites which are typically referred to as Pico or Micro Cells compared to the larger higher power macro cell base stations.
Two key challenges of cell splitting are site acquisition and backhaul.
Considering site acquisition, for macro-cells, the ability to cell split is restricted by the number of available towers or high-rise buildings. Furthermore, the current lease rates on a tower or high-rise building can easily run at $2 k per month or $24 k per year in developed economies. As an operator cell splits, the number of cell lease agreements and his resultant operational expenses (OPEX) fees increase proportionally. Furthermore, zoning laws may restrict the ability to build new towers and in some jurisdictions even if they allow a new tower to be built, obtaining a permit can take several years.
PicoCells offer a potential way around the site acquisition issue. As the power of the BTS and the cell radius decrease into the MicroCell or PicoCell range, the BTS can be deployed at lower elevations, for example on a utility pole. In the US the FCC has mandated that wireless operators must be given access to utility poles at a predetermined rate to facilitate this industry trend.
With respect to backhaul challenges, a 4G cell site must support data rates which will peak in the range of 100 Mbps with average data rates perhaps in the range of 10 Mbps. Peak data rates of 100 Mbps are currently only supported by fiber or by Microwave radio links. High capacity fiber links are available on major high rise buildings and on many cellular towers, but they are not available for the vast majority of utility poles where an operator may wish to deploy a PicoCell. Furthermore, supporting the peak data rates that a 4G cell site will be able to generate necessitate the operator to equip each PicoCell with a link capable of supporting a similar backhaul speed. Today, a 100 BaseT Ethernet link can cost upwards of $1500/month in the US and Canada, which results in very significant OPEX, costs ($18 k/year). If an operator decided to reduce on backhaul costs by equipping his PicoCells with DSL or Cable Modems, then the Peak data rates that can be supported will be greatly diminished and the user experience and the operator's competitive position is reduced.
Microwave radio is a cost effect means of providing a high capacity backhaul connection. A typical Microwave radio link can be installed for a one time cost of approximately $10K and recurring OPEX fee of about $2 k/link/year to the owner of the spectrum. Microwave radios can be deployed to provide a high capacity backhaul link from the BTS to an aggregation point where a high capacity fiber link is available. Given that a GigE link is only marginally more expensive than a 100 BT link, the ability to aggregate traffic to a common location provides significant savings. This is considerably cheaper than leasing a 100 BT fiber link for each BTS. The complication is that Microwave Radio operates at higher frequencies and as a result is restricted to Line of Sight (LOS) type deployments. This is not a major impediment for establishing a link between two elevated sites, which are substantially above the clutter, but it is no longer an option when the PicoCells or Microcells are deployed on lower elevation structures, below clutter, and LOS conditions no longer exist between the PicoCell and a desired aggregation point.
Thus, although cell splitting, with deployment of Microcells and PicoCells, offers advantages in increasing cell site capacity, current LOS solutions for wireless backhaul require that cell sites and aggregations points (BTS) are elevated, above the clutter. Thus backhaul remains a bottleneck for 4G, and to some extent, 3G networks. Thus it would be desirable to provide a NLOS backhaul solution, which would be capable of providing cost effective, high capacity connection/link from a Base Station (MicroCell or PicoCell) to a common aggregation point. On the other hand, there are a number of other challenges that arise in implementing a NLOS solution.
LOS Microwave antenna can be highly directional, reducing the probability of co-channel interference to a low value. NLOS Radio Links operate at lower frequencies than LOS Microwave Radio Links, and a larger path loss is expected for a given propagation distance because the signal must travel through obstructions such as buildings, trees, or around small hills. Reduced directionality, the random nature of obstructions, fluctuating path losses and beam spreading increase the probability of co-channel interference. Effective deployment of NLOS backhaul solutions therefore requires control of Carrier to Interference and Noise Ratio (CINR).
Furthermore, the availability of spectrum at lower frequencies, which can be used to implement NLOS backhaul links is scare and as such the channel bandwidth is typically limited to 10 MHz or 20 MHz whereas for a microwave link operating at higher frequencies, larger channel bandwidth of 40 Mhz or even 50 MHz are typical. As such, to effectively implement high capacity networks employing NLOS backhaul, spectral efficiency and an aggressive frequency re-use is important.
Beam forming techniques represent a promising method to increasing the frequency reuse pattern of a wireless network and thereby increase the overall capacity of the network. Beam forming has been the object of research and trials in 2G and 3G networks but has never seen widespread use due to deployment challenges. One of the largest challenges has been the size of the antenna panel which is needed to create a multi-beam system and the resultant deployment issues. Typically, a conventional sector antenna at 2.5 GHz, as represented schematically in FIG. 1, with 17 dBi of gain, will be approximately 36 inches high by 8 inches wide. These dimensions would be for a single column, with potentially two polarizations. The resultant beam pattern would have a 3 dB beam width of approximately 60 degrees in the azimuth plane and perhaps 10 degrees in the elevation plane. In order to implement a 6 beam antenna, six such antenna columns would be places on a panel as show schematically in FIG. 2, and the resultant dimensions of the panel would grow to 36 inches high, by 40 inches wide. The associated weight and wind loading of the antenna would be approximately 5 times larger. The benefit to the system designer is two-fold. Firstly, as discussed previously the antenna would now be able to create 8 distinct beams within a single sector and hence the frequency reuse pattern of the cell site can be increased, which results in a much higher capacity. The second benefit is that the system gain of the combined antenna would be about 9 dB higher than for the single column antenna, so 26 dBi, as opposed to 17 dBi for the single column antenna.
From a deployment perspective there are several issues with the large panel antenna needed to implement beam forming:                a) The size of the antenna results in significantly more wind loading on the tower than a sector antenna. Cellular towers that were originally engineered to withstand a certain amount of wind loading may not be able to support this new larger antenna.        b) The large antenna is an eyesore and it is more difficult for operators to obtain a permit to deploy such a large antenna panel.        c) Historically there have been large and expensive RF cables between the Base Station Transceiver which is on the ground and the antenna. Beam forming systems require a radio to be connected to each antenna column and hence there is a significant increase in the cost, size and weight of the RF cables.        
For NLOS backhaul, given that the links are implemented between two stationary nodes, beam forming is a potentially attractive alternative to increase the frequency reuse and the overall network capacity.
An object of the present invention is to provide a wireless backhaul solution which uses beam forming and addresses at least some of the above-mentioned issues in implementing cell splitting, particularly for deployment of Microcells and PicoCells for wireless backhaul.